Oxides of nitrogen (hereinafter referred to as NOx) which may risk the environment are contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or the like. As an exhaust gas purification system used for purifying NOx there has been known an exhaust gas purification system (SCR system) for injecting and mixing urea solution as reducing agent into exhaust gas and selectively reducing and purifying NOx with catalyst.
Here, in the case where urea solution is used as reducing agent, hydrolysis occurs in the urea solution and thus it is crystallized when the urea solution is kept under a predetermined temperature range, so that clogging may occur in a reducing agent supply passage or an injection nozzle. Therefore, there has been disclosed a device and a method which use an injector 432 as means for supplying urea solution into an exhaust gas passage 420, and comprise means for supplying urea solution from a stock tank 430 through a line 431 into the injector 432, and means for returning the urea solution from the injector 432 to the stock tank 430 as shown in FIG. 8, whereby the urea solution is returned from the injector 432 to the stock tank 430 by a supply speed and a returning speed which are enough to keep the temperature of the urea solution sufficiently low, thereby reducing NOx exhaust (see Patent Document 1).
Patent Document 1: JP-T-2001-518830 (the whole text all figures).